Interior welding of tubes or other conduits to repair defects or wear within the walls is a necessary and time-consuming task for owners and operators of processes and equipment in which high pressure fluid is transferred. Although many techniques have been developed and used in the prior art for welding of installed tubing, effective interior tube welding is still a challenge to those in the industry. This is due primarily to the difficulty in accessing sections of the installed tubing which may be within a tube bank or pressure vessel. Moreover, since significant structural degradation is likely to have occurred after an extended period of operation under conditions of high pressure, high temperature, and/or corrosive environment, leaking tubes often require the installation of internal liner sleeves with full circumferential welds at each end of the sleeve.
One such application wherein structural integrity of a repaired or "sleeved" tube is critical is in the steam generators of a nuclear powered electric generating plant. In such a plant, the heated primary coolant flows through a plurality of inverted U-shaped tubes immersed in water from which steam is generated.
As it is typical in such an application for the primary side coolant pressure to be higher than that of the secondary, steam producing side, it is apparent that a failure of a steam generator tube or a sleeve repair in such a tube will cause a leakage from the primary coolant into the secondary system.
The occurrence of such leakage is not usual in such units, especially after extended periods of operation. For this reason, welded repairs on the individual steam generator U tubes are performed at regular intervals while the plant is shut down for service.
One welding device for this purpose is the subject of U.S. Pat. No. 4,580,028, issued to Kobuck et al on Apr. 1, 1986. This device, however, has a limited use because it is not fully receivable in a tube to be sleeved or repaired since the rotational drive mechanism must remain outside of the tube. Moreover, in any device in which the weld head is attached to the top of a rigid shaft and the lower end of the shaft has an electric motor that turns the weld head tip, the length of the shaft is the limit of height into a nuclear steam generator tube at which a weld can be made. This height is positively limited by the distance from the lower or tube sheet end of the tube to the bottom of the steam generator because of the rigid drive shaft. To place and weld sleeves at locations high in the steam generator, a flexible shaft for driving the device is, therefore, required. There is a problem in turning a flexible shaft smoothly which has motivated the instant invention in which only the top of the weld head is rotated.
Accordingly, it would be useful to have a welding apparatus of simple and reliable design having a flexible drive which turns only a top weld head and is receivable in the tube for both circumferentially and longitudinally traversing the interior of a nuclear steam generator tube, or the like, to be "sleeved" or otherwise repaired by welding. A simple, low cost rotating head for mounting the weld tip and providing it with a flexible drive shaft, electricity and a supply of inert gas, accordingly, is provided. With the sleeving of failed tubes currently presenting an economically and technically beneficial alternative to the former practice of simply plugging off the failed tube in a nuclear steam generator, the need for such a welding apparatus is readily apparent to those skilled in the art.